According to the known processes for isomerization of aromatic compounds with eight carbon atoms, a feedstock that is generally low in paraxylene relative to the thermodynamic equilibrium of the mixture (i.e., whose paraxylene content is clearly less than that of the mixture with the thermodynamic equilibrium at the temperature in question, whereby this mixture comprises at least one compound that is selected from the group that is formed by metaxylene, orthoxylene, paraxylene and ethylbenzene) and generally rich in ethylbenzene relative to this same mixture in thermodynamic equilibrium is introduced into a reactor that contains at least one catalyst under suitable temperature and pressure conditions to obtain a composition, at the outlet of said reactor, of aromatic compounds with eight carbon atoms that is as close as possible to the composition of said mixture in thermodynamic equilibrium at the temperature of the reactor.
Paraxylene and optionally orthoxylene, which are the desired isomers because they exhibit an important advantage particularly for the synthetic fiber industry, are then separated from this mixture. Metaxylene and ethylbenzene can then be recycled to the inlet of the isomerization reactor so as to increase the production of paraxylene and orthoxylene. When it is not desired to recover orthoxylene, the latter is recycled with metaxylene and ethylbenzene.
The isomerization reactions of the aromatic compounds with eight carbon atoms per molecule pose, however, several problems that are produced by secondary reactions. Thus, in addition to the main isomerization reaction, hydrogenation reactions are observed, such as, for example, the hydrogenation of the aromatic compounds to naphthenes, reactions of opening naphthene cycles that lead to the formation of paraffins that have at most the same number of carbon atoms per molecule as the naphthenes from which they are obtained. Cracking reactions are also observed, such as, for example, the cracking of paraffins that lead to the formation of light paraffins that typically have from 3 to 5 carbon atoms per molecule, dismutation and transalkylation reactions that lead to the production of benzene, toluene, aromatic compounds with nine carbon atoms per molecule (trimethylbenzenes, for example) and heavier aromatic compounds.
All of these secondary reactions are greatly detrimental to the yields of desired products.
The amount of secondary products that are formed (naphthenes that typically contain from 5 to 8 carbon atoms, paraffins that contain from 3 to 8 carbon atoms, benzene, toluene, aromatic compounds with, for the most part, 9 and 10 carbon atoms per molecule) depends on the nature of the catalyst and the operating conditions of the isomerization reactor (temperature, partial hydrogen and hydrocarbon pressures, feedstock flow rate).
It is well known to one skilled in the art that the secondary reactions increase when the paraxylene content in the reactor is closer to the paraxylene content in thermodynamic equilibrium under given pressure and temperature conditions.
The optimization of the operating conditions as well as the optimization of the formulation of the isomerization catalyst make it possible to improve the paraxylene yield but not to be loss-free. In addition, the search for obtaining new catalysts is a long and costly activity.
We have discovered, in a surprising way, that it is possible to reach paraxylene contents that are close to the paraxylene content in thermodynamic equilibrium while reducing the xylene losses by combining at least two reaction stages.